The core of a nuclear reactor comprises a plurality of nuclear fuel assemblies, each assembly consisting of a plurality of nuclear fuel rods. Each fuel rod comprises a circular cylindrical housing, i.e., cladding, which is sealed at both ends by respective end plugs. A plurality of nuclear fuel pellets are stacked in a vertical column inside the cladding to a height less than the length of the cladding, leaving a plenum space above the fuel column. A compression spring is placed inside the plenum for biasing the fuel pellets toward the bottom end plug of the fuel rod. A getter for removing contaminants from the interior atmosphere is conventionally installed inside the plenum.
The cladding serves two primary purposes: first, the cladding prevents contact and chemical reaction between the nuclear fuel and the coolant/moderator; and second, the cladding prevents the radioactive fission products, some of which are gases, from being released from the fuel rod into the coolant/moderator. Failure of the cladding, due to build-up of gas pressure or any other reason, could result in contamination of the coolant/moderator and associated systems by radioactive long-lived products to a degree which would interfere with plant operation.
A conventional technique for identifying defective fuel elements in water-moderated nuclear reactors is known as "fuel sipping". This technique identifies leaking fuel rods by obtaining and measuring fission gases that leak out of defective fuel rods. A known method and apparatus for performing fuel sipping is disclosed in U.S. Pat. No. 4,034,599, assigned to the present assignee, the disclosure of which is incorporated by reference herein. In accordance with this conventional technique, fuel sipping is accomplished by isolating a fuel assembly in a test chamber of purified water. The test chamber may be located either in the reactor vessel or at the bottom of the fuel pool. A fuel assembly or a fuel rod is removed from the core or fuel storage rack and placed in a closed container, the background is diminished, and a gas sample which is released from the fuel assembly or rod is obtained from the sipping container. The test chamber contains an exhaust line near the top and a gas sparger at the bottom. Air is introduced into the test chamber through the gas sparger and is allowed to displace a portion of the water above the fuel element. This serves to form an air pocket above the fuel element, reduce the pressure in the test chamber and simultaneously purge the water surrounding the fuel element of fission gases pulled from defective fuel elements. The activity of fission gases entrained in the air are then measured by passing the air through a suitable radiation monitor. In a second step in the method, the pressure in the test chamber is further reduced to a vacuum, so as to increase the release of fission gases. In a third step of the method, the pressure in the test chamber is held at a vacuum and gas drawn from the air pocket above the fuel element for testing is recirculated so as to continuously purge released fission gases from the water surrounding the fuel element. In this manner, purge air and fission gas are trapped in the air pocket in the top of the test chamber and are removed for monitoring via a sample line. The radiation monitor in accordance with U.S. Pat. No. 4,034,599 is a gross beta detector. This detector simultaneously responds to both Kr-85 and Xe-133, which are the major fraction of the fission gases. This system is very accurate, but slow in determining if a slow leaker is present, due to the need to remove the fuel elements from the reactor and due to the time-consuming nature of the detection process.
The measurement of fission gases is a key element of the fuel sipping process because of the easily achieved separation of gas and water. However, the Xe-133 isotope is a decay product of I-133, which is a water-soluble ion. In the case of the aforementioned vacuum sipper, this results in a background problem which is minimized by using demineralized make-up water. Demineralized condensate cannot be used because it often causes problems due to the release of Xe-133 from the decay of I-133 which has been carried over in the steam and exchanged on the condensate demineralizers. Pool water has large quantities of I-133 uniformly distributed therein. The concentration of I-133 is greatly increased when fuel pellet material escapes through a defect in the fuel rod cladding. These background problems must be considered when a slow leaker is observed. In this case, a small increase in fission gas is indicated during the gas recirculation mode of a fuel sip using the vacuum sipper. This increase could be due to a very small defect in a rod, pool water leaking into the test chamber, or desorption of gas from the oxide film which may also contain I-133 (chemically bound). This is a problem because it can lead to false identifications of a leaking fuel rod.
The Kr-85 isotope should not present this type of background problem because there are no water-soluble ionic species in its decay scheme. Therefore, once any species migrates from the inside to the outside of the fuel rod, it will separate and be swept away. It should be noted that Kr-85 is not nearly as abundant as Xe-133. The only technique used to determine the quantity of Kr-85 in the presence of Xe-133 is to make repeated measurements of a specific sample over a long period to determine the decay characteristics of the mixture and calculate the respective quantities of Kr-85 and Xe-133 based on the decay half-lives. This measurement procedure can take months to complete. The rapid measurement of Kr-85 (exclusively) reduces or eliminates a false positive response in the fuel sipping process.